Question

在Ruby中，如何从该类的实例中调用类方法？说我有

class Truck
  def self.default_make
    # Class method.
    "mac"
  end

  def initialize
    # Instance method.
    Truck.default_make  # gets the default via the class's method.
    # But: I wish to avoid mentioning Truck. Seems I'm repeating myself.
  end
end

行Truck.default_make检索默认值。但有没有提到Truck的方法呢？好像应该有。

Answer 1

在实例方法中，您只需调用self.class.whatever，而不是引用类的文字名称。

class Foo
    def self.some_class_method
        puts self
    end

    def some_instance_method
        self.class.some_class_method
    end
end

print "Class method: "
Foo.some_class_method

print "Instance method: "
Foo.new.some_instance_method

输出：

Class method: Foo
Instance method: Foo

Answer 2

使用self.class.blah与继承时使用ClassName.blah不同。

class Truck
  def self.default_make
    "mac"
  end

  def make1
    self.class.default_make
  end

  def make2
    Truck.default_make
  end
end


class BigTruck < Truck
  def self.default_make
    "bigmac"
  end
end

ruby-1.9.3-p0 :021 > b=BigTruck.new
 => #<BigTruck:0x0000000307f348> 
ruby-1.9.3-p0 :022 > b.make1
 => "bigmac" 
ruby-1.9.3-p0 :023 > b.make2
 => "mac"

Answer 3

要访问实例方法中的类方法，请执行以下操作：

self.class.default_make

以下是您的问题的替代解决方案：

class Truck

  attr_accessor :make, :year

  def self.default_make
    "Toyota"
  end

  def make
    @make || self.class.default_make
  end

  def initialize(make=nil, year=nil)
    self.year, self.make = year, make
  end
end

现在让我们使用我们的课程：

t = Truck.new("Honda", 2000)
t.make
# => "Honda"
t.year
# => "2000"

t = Truck.new
t.make
# => "Toyota"
t.year
# => nil

Answer 4

self.class.default_make

Answer 5

你正在以正确的方式做到这一点。类方法（类似于C ++或Java中的'静态'方法）不是实例的一部分，因此必须直接引用它们。

就此而言，在您的示例中，您可以更好地使'default_make'成为常规方法：

#!/usr/bin/ruby

class Truck
    def default_make
        # Class method.
        "mac"
    end

    def initialize
        # Instance method.
        puts default_make  # gets the default via the class's method.
    end
end

myTruck = Truck.new()

类方法对于使用该类的实用程序类型函数更有用。例如：

#!/usr/bin/ruby

class Truck
    attr_accessor :make

    def default_make
        # Class method.
        "mac"
    end

    def self.buildTrucks(make, count)
        truckArray = []

        (1..count).each do
            truckArray << Truck.new(make)
        end

        return truckArray
    end

    def initialize(make = nil)
        if( make == nil )
            @make = default_make()
        else
            @make = make
        end
    end
end

myTrucks = Truck.buildTrucks("Yotota", 4)

myTrucks.each do |truck|
    puts truck.make
end

Answer 6

如果您有权访问委托方法，则可以执行以下操作：

[20] pry(main)> class Foo
[20] pry(main)*   def self.bar
[20] pry(main)*     "foo bar"
[20] pry(main)*   end  
[20] pry(main)*   delegate :bar, to: 'self.class'
[20] pry(main)* end  
=> [:bar]
[21] pry(main)> Foo.new.bar
=> "foo bar"
[22] pry(main)> Foo.bar
=> "foo bar"

或者，如果你想要委托给一个或多个方法，你可能会更加清晰。实例：

[1] pry(main)> class Foo
[1] pry(main)*   module AvailableToClassAndInstance
[1] pry(main)*     def bar
[1] pry(main)*       "foo bar"
[1] pry(main)*     end  
[1] pry(main)*   end  
[1] pry(main)*   include AvailableToClassAndInstance
[1] pry(main)*   extend AvailableToClassAndInstance
[1] pry(main)* end  
=> Foo
[2] pry(main)> Foo.new.bar
=> "foo bar"
[3] pry(main)> Foo.bar
=> "foo bar"

提醒：

不要随意delegate所有不会将状态更改为类和实例的内容，因为您将开始遇到奇怪的名称冲突问题。谨慎地做到这一点，只有在你检查后没有别的东西被压扁。

Answer 7

还有一个：

class Truck
  def self.default_make
    "mac"
  end

  attr_reader :make

  private define_method :default_make, &method(:default_make)

  def initialize(make = default_make)
    @make = make
  end
end

puts Truck.new.make # => mac

Answer 8

以下是针对此情况实施_class方法的方法，该方法适用于self.class。注意：不要在生产代码中使用它，这是为了感兴趣：）

来自：Can you eval code in the context of a caller in Ruby?以及http://rubychallenger.blogspot.com.au/2011/07/caller-binding.html

# Rabid monkey-patch for Object
require 'continuation' if RUBY_VERSION >= '1.9.0'
class Object
  def __; eval 'self.class', caller_binding; end
  alias :_class :__
  def caller_binding
    cc = nil; count = 0
    set_trace_func lambda { |event, file, lineno, id, binding, klass|
      if count == 2
        set_trace_func nil
        cc.call binding
      elsif event == "return"
        count += 1
      end
    }
    return callcc { |cont| cc = cont }
  end
end

# Now we have awesome
def Tiger
  def roar
    # self.class.roar
    __.roar
    # or, even
    _class.roar
  end
  def self.roar
    # TODO: tigerness
  end
end

也许正确的答案是为Ruby提交补丁：）

Answer 9

与您的问题类似，您可以使用：

class Truck
  def default_make
    # Do something
  end

  def initialize
    super
    self.default_make
  end
end

Ruby：从实例调用类方法

9 个答案: